1. Field of the Invention
This invention relates generally to the structure and fabrication process of an organic light emitting diode. More particularly, this invention relates to an improved layer structure and method of manufacture for an organic light-emitting LED device applicable for flat panel display applications with extended operational life and reliable light-emitting performance.
2. Description of the Prior Art
A technical difficulty is still confronted by the manufacturers of flat panel devices because a long-term reliable operation of the organic light-emitting diodes (LED) is not yet achievable. For the purpose of forming an organic LED, an active organic luminescent layer is sandwiched between a transparent anode, e.g., an indium/tin oxide (ITO) layer, as a hole-injecting contact and a low work function metal as electron injecting contacts, as that described below as that shown in FIG. 1. The active organic layer may be a conductive polymer. However, there is a difficulty faced by those applying the organic LEDs for display applications, because the operational life for a typical organic LED is still quite limited. Due to the difficulties of layer structure integrity, as will be further explained, degradation of display quality and reduction in brightness often occur after the organic light emitting device is deployed for service for only a limited period of time.
FIG. 1A shows the layer structure of a conventional light emitting diode. An organic light emitting layer 3 is deposited over an indium/tin oxide (ITO) layer 2 supported on a glass substrate 1. The ITO layer 2 serves the function as anode with a metal layer 4 overlying the organic light-emitting layer 3 functioning as a cathode electrode connected to a negative voltage of three to nine volts (xe2x88x923V to xe2x88x929V) to induce the light emitting function of the layer 3. The light emitting organic layer 3 can be a single layer or a multiple-layer structure with a thickness less than 100 nm. Because of this very small thickness, the light-emitting organic layer is vulnerable to manufacture defects of pinholes or damages when dust particles incidentally settle over the top or underneath the light-emitting organic layer 3. The anode electrode, i.e., the ITO layer 2 is shorted to the cathode-electrode metal layer 4 when the pinholes are presented in the light-emitting layer 3. Malfunctions of the light-emitting flat panel device become a difficult problem faced by those of ordinary skill in the art involved in making display devices by applying techniques of light-emitting diode implemented with organic light emitting layers of very small layer thickness.
Zhang et al. disclose in an U. S. Pat. No. 5,798,170, entitled xe2x80x9cLong Operating Life for Polymer Light Emitting Diodesxe2x80x9d a light emitting diode with improved resistance to blurring spreading or degradation over time.
As shown in FIGS. 1B and 1C, a polymer light-emitting diode device 20 is disclosed in this patent that includes an electronic injecting cathode contact 12 that can be a relatively low work function metal such as calcium. The cathode contact 12 is disposed on top of semi-conductive and luminescent conjugated polymer film 14 supported on a substrate 18. The substrate 18 is partially coated with a transparent conductive layer 16 having a higher work function, i.e., a high ionization potential to serve as the electron withdrawing anode electrode. The light emitting diode configuration is then improved by placing a layer 15 containing polyaniline, i.e., PANI, emeraldine salt to interpose between the luminescent film 14 and electrode 16. FIG. 1C shows an alternate embodiment of the patented device 10 by employing the PANI layer 15 as the electron withdrawing anode electrode wherein the PANI layer 15 contains emeraldine salt of high conductivity. Improved performance over a longer life is achieved because the emeraldine salt containing layer 15 with certain sheet resistance can stabilize the device performance and prevent degradation of efficiency loss in forming the dark non-emitting spots.
The usefulness of employing a polyaniline layer containing various level of resistance of emeraldine-salt, as disclosed by Zhang et al., is however limited by the difficulties that PANI layer is not suitable to function as an intermediate layer. The difficulties arise from the facts high level of uniformity of PANI layer thickness and density distribution cannot be precisely controlled. Problems of pinhole damages may still occur. Due to these difficulties, even with a PANI layer as suggested by Zhang et al., the problems of short-circuit between the anode and cathode and device malfunctions cannot be totally prevented.
Therefore, a need still exists in the art of design and manufacture of flat panel display by employing the organic light emitting diodes. More specifically, a need still exists in the art to provide an organic light-emitting diode with a layer structure that can reduce the vulnerability to the pinhole problems. Such layer structure must also provide a current limiting capability for protecting the device from damages due to a large current conducting between the anode and cathode. Moreover, the light-emitting device further requires a current distribution layer to reduce the uneven distributions of current densities such that the degradation of device performance caused by uneven current density distributions can be minimized.
It is therefore an object of the present invention to provide a new layer structure and manufacture method for manufacturing an organic light-emitting diode capable of reliable long-term operation without degradation. The vulnerability to pinhole problems and uneven current distributions are reduced such that aforementioned difficulties and limitations encountered in the prior art can now be overcome.
Specifically, it is an object of the present invention to provide an improved layer structure with a new intermediate inorganic layer that has strong resistance to layer damages caused by fine particles such that the improved layer structure can reduce the vulnerability to pinholes problems.
Another object of the present invention is to provide a new and improved layer structural for an organic light-emitting diode with an inorganic intermediate layer serving as a current limiting layer. Device damages caused by current exceeding a maximum current density are prevented.
Another object of the present invention is to provide a new and improved layer structural for an organic light-emitting diode with an inorganic intermediate layer serving as a current distribution layer. Device performance degradation caused by uneven current density distribution after a long term operation is reduced by more evenly distributing the current density over the entire surface of the light-emitting diode.
Briefly, in a preferred embodiment the present invention includes an organic light-emitting diode (LED). The descriptions below are denoted with numeral designations, which are depicted in FIG. 2 described below. The organic light emitting diode is supported on an indium/tin oxide 110 (ITO) coated glass substrate 105. The organic light-emitting diode includes an amorphous-silicon (xcex1-Si) resistive layer 115 covering the ITO 110 coated glass substrate 105. The organic light-emitting diode 100 further includes a polyaniline (PANI) layer 120 covering the amorphous silicon (xcex1-Si) resistive layer 115 and an organic light emitting layer 125 overlying the PANI layer 120. And, the organic light-emitting diode 100 further has a conductive electrode layer 130 covering the light emitting layer 125. In a preferred embodiment, the amorphous silicon (xcex1-Si) resistive layer 115 functioning as a current limiting layer for limiting a current density conducted between the ITO 110 coated glass substrate 105 and the conductive electrode layer 130 under a maximum allowable current density of 1000 mA/cm2. In another preferred embodiment, the amorphous silicon (xcex1-Si) resistive layer 115 functioning as a current distribution layer for distributing a current conducted between the ITO coated glass substrate and the conductive electrode layer. Thus, the difference between a greatest current density from a smallest current density is under a maximum allowable current density difference of 1000 mA/cm2. In summary, this invention discloses an organic light-emitting diode (LED) 100 that includes an inorganic layer 115 functioning as a current limiting layer.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.